Christian Hühne

Probabilistic Design of Axially Compressed Composite Cylinders with Geometric and Loading Imperfections

The discrepancy between the analytically determined buckling load of perfect cylindrical shells and experimental test results is traced back to imperfections. The most frequently used guideline for design of cylindrical shells, NASA SP-8007, proposes a deterministic calculation of a knockdown factor with respect to the ratio of radius and wall thickness, which turned out to be very conservative in numerous cases and which is not intended for composite shells. In order to determine a lower bound for the buckling load of an arbitrary type of shell, probabilistic design methods have been developed. Measured imperfection patterns are described using double Fourier series, whereas the Fourier coefficients characterize the scattering of geometry. In this paper, probabilistic analyses of buckling loads are performed regarding Fourier coefficients as random variables. A nonlinear finite element model is used to determine buckling loads, and Monte Carlo simulations are executed. The probabilistic approach is used for a set of six similarly manufactured composite shells. The results indicate that not only geometric but also nontraditional imperfections like loading imperfections have to be considered for obtaining a reliable lower limit of the buckling load. Finally, further Monte Carlo simulations are executed including traditional as well as loading imperfections, and lower bounds of buckling loads are obtained, which are less conservative than NASA SP-8007.

The use of topology optimisation in the conceptual design of next generation lattice composite aircraft fuselage structures

Conventional commercial aircraft fuselages use all-aluminium semi-monocoque structures where the skin carries the external loads, the internal fuselage pressurisation and is strengthen using frames and stringers. Environmental and economic issues force aircraft designers to minimise weight and costs to keep air transport competitive and safe. But as metal designs have reached a high degree of perfection, extraordinary weight and cost savings are unlikely in the future. Carbon composite materials combined with lattice structures and the use of topology optimisation have the potential to offer such weight reductions. The EU FP7 project Advanced Lattice Structures for Composite Airframes (ALaSCA) was started to investigate this. This article present some of this research which has now led to the development of a new airframe concept which consists of: a load carrying inner skin; transverse frames; CFRP-metal hybrid stiffeners helically arranged in a grid configuration; insulating foam and an additional aerodynamic outer skin.

Deployable Composite Booms for Various Gossamer Space Structures

Deployable structures are required to either enable orbit transfer of very large structure or to make the orbit transfer of medium and small size structures more affordable. Hereby, deployable booms are basic building blocks of such deployable structures. DLR is providing a concept for deployable booms that utilize very thin CFRP material and which can be stowed by coiling. The given paper introduces the concept of the CFRP booms and discusses the problems of their self deployment tendency. Furthermore, different mechanisms are presented that are able to control the deployment. Tests under artificial zero-g environment have been conducted to verify the applicability of the control concepts. Hence, the paper also gives insight in objectives, setup and the results of the experiment as well as a final evaluation of the concepts. Finally, an outlook on current and future projects that use the introduced booms or equivalent systems is given.

Material and Failure Models for Textile Composites

The complex three-dimensional structure of textile composites makes the experimental determination of the material parameters very difficult. Not only the number of constants increases, but especially through-thickness parameters are hardly quantifiable. Therefore an information-passing multiscale approach for computation of textile composites is presented as an enhancement of tests, but also as an alternative to tests. The multiscale approach consists of three scales and includes unit cells on micro- and mesoscale. With the micromechanical unit cell stiffnesses and strengths of unidirectional fiber bundle material can be determined. The mesomechanical unit cell describes the fiber architecture of the textile composite and provides stiffnesses and strengths for computations on macroscale. By comparison of test data and results of numerical analysis the numerical models are validated.

A modular approach to adaptive structures.

A remarkable property of nastic, shape changing plants is their complete fusion between actuators and structure. This is achieved by combining a large number of cells whose geometry, internal pressures and material properties are optimized for a given set of target shapes and stiffness requirements. An advantage of such a fusion is that cell walls are prestressed by cell pressures which increases, decreases the overall structural stiffness, weight. Inspired by the nastic movement of plants, Pagitz et al (2012 Bioinspir. Biomim. 7) published a novel concept for pressure actuated cellular structures. This article extends previous work by introducing a modular approach to adaptive structures. An algorithm that breaks down any continuous target shapes into a small number of standardized modules is presented. Furthermore it is shown how cytoskeletons within each cell enhance the properties of adaptive modules. An adaptive passenger seat and an aircrafts leading, trailing edge is used to demonstrate the potential of a modular approach.

Testing of the Deorbitsail drag sail subsystem

Deorbitsail is a 3U Cubesat project that will launch, deploy, and support a large drag sail deorbiting payload. The flight payload sail system will deploy to 5 by 5 meters, increasing the frontal area of the spacecraft dramatically. A series of 4-by-4-meter sail deployments has been performed with a prototype of the proposed system. The prototype system tests included partial deployments in environmental chambers and full-scale deployment at ambient lab conditions. Design changes arising from testing of the sail system prototype are described in this paper. Engineering model construction is underway and the Deorbitsail launch will be in 2014. Deorbitsail is a European Commission 7th Framework Programme project with nine partner organizations.

Detailed design of a lattice composite fuselage structure by a mixed optimization method

In this article, a procedure for designing a lattice fuselage barrel is developed. It comprises three stages: first, topology optimization of an aircraft fuselage barrel is performed with respect to weight and structural performance to obtain the conceptual design. The interpretation of the optimal result is given to demonstrate the development of this new lattice airframe concept for the fuselage barrel. Subsequently, parametric optimization of the lattice aircraft fuselage barrel is carried out using genetic algorithms on metamodels generated with genetic programming from a 101-point optimal Latin hypercube design of experiments. The optimal design is achieved in terms of weight savings subject to stability, global stiffness and strain requirements, and then verified by the fine mesh finite element simulation of the lattice fuselage barrel. Finally, a practical design of the composite skin complying with the aircraft industry lay-up rules is presented. It is concluded that the mixed optimization method, combining topology optimization with the global metamodel-based approach, allows the problem to be solved with sufficient accuracy and provides the designers with a wealth of information on the structural behaviour of the novel anisogrid composite fuselage design.

The Design and Test of the GOSSAMER-1 Boom Deployment Mechanisms Engineering Model

The main focus of this paper is the detailed introduction of the oom deployment concept implemented for the German eployable membrane technology demonstrator Gossamer-1. The technology aims for solar sailing, thin-film photovoltaic and drag augmentation as possible use cases. Therefore, the main functional and geometrical requirements for the mechanisms are derived from the mission design and the global sail deployment concept which are both introduced in detail. The regarding mechanism design is explained on concept level and illustrated with evaluating tests of the engineering model. Finally, an outlook on the future of the project, including the current design status of the engineering qualification model, is given.

Structural Optimization of Composite Wings in an automated Multi-Disciplinary Environment

This paper presents a structure design and optimization module, developed for the application inside multi-disciplinary optimization process chains. The loads necessary for sizing and optimization are calculated in CFD or aeroelastic calculations and applied on a Finite Element Model that represents all primary structural elements of a wing. The FE model is created automatically from a parametric geometry description. The deformations and inner loads of the wingbox are calculated via linear static FE calculations; geometry and loads are provided to an external sizing tool. Each component has a set of design variables with discrete design points which are permutated to get the component’s design candidates. A set of failure criteria is used to size the structure which can be made of composites or metal. The methodology is applied to the optimization of a forward swept composite for a short range aircraft and a design study comparing different stringer types and their influence on mass and structural deformation of the wing is performed.

Design and Sizing of the GOSSAMER Boom Deployment Concept

Since 2011 DLR is preparing 3 orbit demonstrations of deployment and operation of a solar sail. For this mission bundle a new deployment control mechanism for DLR’s tubular CFRP shell booms has been developed that is applicable to all 3 evolution steps of the GOSSAMER—Road Map. According to the mission goal of GOSSAMER-1 and its addressed deployed sail size of 5 × 5 m2, a down scaled configuration of the booms and an adapted deployment mechanism are currently under development. The paper introduces the boom and its deployment mechanism concepts and describes and concludes tests and simulations that have been performed to proof the mechanical performance of the deployed booms.